It has long been known that ear tags impregnated with insecticide are an effective means for providing control of horn flies and other insects on domestic livestock, particularly cattle. The most common material for formulating insecticidal animal tags has heretofore been polyvinyl chloride. In recent years, concern from ecological and human safety viewpoints have been raised with regard to utilizing plasticized polyvinyl chloride on animals intended for human consumption. Also, since polyvinyl chloride must be plasticized to perform satisfactorily as a tag, the quantity of active insecticide which can be incorporated into the product is reduced in direct proportion to the quantity of plasticizer required.
Another disadvantage of present insecticidal tag formulating techniques is that the high loadings of plasticizer and insecticide cause the completed product to "bleed" (exude) insecticide. This makes an unacceptable commercial product which requires special packaging to accommodate a relatively long shelf life, necessitates avoiding high temperatures in transport and storage, and demands special handling requirements when the product is removed from the package for application to an animal.
It has also been previously known to utilize polyurethane resins for forming identification (I.D.) tags for animals. It is also known to use polyurethane I.D. tags and attach to same a porous or semi-permeable membrane in the form of a cell for holding a reservoir of insecticide. These membranes are made of a variety of different polymers including polyurethane. Their construction is shown and described in U.S. Pat. No. 4,562,794. The purported advantage to utilizing such a membrane is to provide greater control over the uniformity of release rate of the insecticide and higher insecticide depletion than can be obtained when the insecticide is within the polymer matrix. Some of the polymer which have heretofore been utilized for tags of the type contemplated by the referenced patent would not, however, permit sufficiently high loadings of insecticide to achieve a satisfactory tag by blending directly with an ectoparasiticidal compound. Inherently, this known construction is costly to manufacture because of the complicated steps of forming and filling the cell as well as making the mechanical fastener for joining the cell to the tag. Also, there is a higher than desirable incidence of product failure because of the membranes being punctured while in use on animals.
The polyurethane resins which are useful in the present invention are polyurethane aromatic polyether elastomers, specifically the polymers which are the reaction product of 4,4'-diphenylmethane diisocyanate, polytetramethylene glycol and 1,4-butanediol. These resins generally conform to Chemical Abstracts Service Registry Number CAS 9018-04-6.